Drug delivery devices for administering single or multiple doses of an injectable medical product are well-known in the art. Generally, such devices have substantially the same purpose as that of an ordinary syringe.
Drug delivery devices of this kind have to meet a number of user-specific requirements. For instance, in the case of those with diabetes, many users will be physically infirm and may also have impaired vision. Therefore, these devices need to be robust in construction, yet easy to use, both in terms of the manipulation of the parts and understanding of its operation by a user. In addition, the dose setting must be easy and unambiguous. Further, where the device is to be disposable rather than reusable, the device should be inexpensive to manufacture and easy to dispose of. In order to meet these requirements, the number of parts and steps required to assemble the device, and the overall number of material types the device is made of should be kept to a minimum.
Typically, the medicinal product to be administered is provided in a cartridge (i.e., a “medicament cartridge”) that has a moveable piston (also commonly referred to as a “bung”, a “stopper” or a “plunger”) mechanically interacting with a piston rod of a drive mechanism of the drug delivery device. By applying thrust to the piston in the distal direction, a certain amount of the medicinal fluid is expelled from the cartridge.
Due to inevitable manufacturing tolerances there may exist axial clearance between a cartridge's piston and the piston rod. Typically, prior to a primary use of the device, a user has to conduct a so-called “priming” of the drug delivery device in order to ensure that the axial clearance is eliminated such that the initial dose set and all subsequent doses set are delivered accurately, and that the amount of the medicinal product is disposed in a predefined way.
Since a self-administering user might be physically infirm, it is desirable to simplify or even to eliminate the need for the user to perform the priming procedure.
Document EP 1 911 479 A1 discloses a dose dispensing device, wherein a piston and a piston rod are electromagnetically or magnetically coupled. However, since the generation of an electromagnetic field requires a supply of electric energy, such a solution is generally not applicable to drug delivery devices of pen-injector type.
Further, document U.S. Pat. No. 6,196,999 B1 discloses a coupling mechanism, wherein a syringe plunger coupling element having the form of a rearwardly extending cylindrical extension is centrally located on a rearward face of a syringe plunger. This coupling element contains an interior T-shaped cavity, wherein the walls of said cavity are knurled to aid the grip of the coupling mechanism. The coupling mechanism is located on the forward end of a plunger drive ram proximate to the syringe plunger and is in the form of two pawls. These pawls are biased away from the plunger drive ram's axis of symmetry by springs. Operation of a motor advances the drive ram forward along its longitudinal axis to move the pawls of the coupling mechanism into engagement with the cylindrical extension of the syringe plunger.
As the advancing pawls initially enter the cavity of the plunger extension, their forward ends are forced toward one another by the walls of the cavity, overcoming the outward bias of the springs. In order to eliminate an initial clearance between the plunger and the drive ram, the pawls have to fully enter the cavity to grip the knurled wall of the cavity. Afterwards, the syringe plunger and drive ram will move in a cooperated motion.